Spring for compression and tension, mainly in axial direction

ABSTRACT

A spring for compression and tension mainly in an axial direction and made of a material having a Young&#39;s modulus of elasticity of at least 39 GPa, such as metal, steel, glass, plastic or reinforced plastic is disclosed. The spring is in the form of a tube or ring of generally constant thickness. In cross-section, each ring is made up of oppositely directed inner and outer U-shaped spring parts with contiguous axial projections. A spring is provided which has minimal elastic hysteresis.

BACKGROUND OF THE INVENTION

The invention relates to a spring for compression and tension, mainly inaxial direction, which spring is made of a material having a Young'smodulus of elasticity E, which is at least 39 GPa, such as metal, steel,glass, plastic or reinforced plastic, the spring being rotationsymmetrical around a symmetry axis and in the form of a tube or a ring,the material thickness of the spring wall between the end sections ofthe spring being practically constant. The spring, in its lengthdirection, comprises at least two mutually integrated spring parts, inradial direction oppositely directed, and in cross section half-wave orU-shaped.

A U-shaped spring is known from German published patent applicationDE-AS 35 36 661, where FIG. 10 shows a rotation symmetrical U-shapedspring, where the wall of the spring has a practically constantthickness of material. The axial ends of this spring loosely abut acylindrical ring, and consequently there must be radial exterior orinterior guiding surfaces to keep the spring in position. Such aninterior or exterior guiding surface gives a friction, when the springmoves, and consequently a certain hysteresis loop.

Known are also springs shaped like bellows, which tightfitting confine atemperature expanding medium or a movement suppressing medium, and acommon purpose for these types of springs are to obtain a flexibility aslarge as possible with a spring resistance as small as possible, sincethe compressive force mainly has to be generated by the medium confinedin the spring.

SUMMARY OF THE INVENTION

The spring according to the present invention is characterized in thateach end section of the spring comprises an annular axial rigidprojection extending between the diameters of the wave crests of the twonearest half wave formed spring parts, through which projection aneutral surface of the spring is passing. By that means it has appearedthat a spring may be obtained with a hysteresis loop substantiallysmaller than the hysteresis loop of U-shaped springs made of the samematerial and having the same thickness of material. Presumably this isdue to the line of the centres of gravity of the axial cross sections ofthe spring passing through or very close to the axial end projections ofthe spring, and thereby simultaneously the neutral surface of the springis passing through or close to the axial end projections of the spring.The neutral surface of the spring is an imaginary surface and may bedefined as a stress neutral or stress balanced surface, in relation towhich the bending stresses occurring radially outside the surface areequal to and opposite directed to the bending stresses occurringradially inside the surface during the increasing, decreasing orstationary load of the spring. This neutral surface of the spring may bea cylinder surface, a cone surface, an hour-glass shaped surface or avase shaped surface depending on the performance of the spring.Compression or tension stresses are of course not zero in the neutralsurface. Due to the fact that the spring transfers its compressive ortensile force at its ends via the axial projections, which are situatedclose to the said neutral surfaces, a spring is obtained, which issubstantially more uniformly balanced than the said known U-shapedspring of DE 35 36 661, and therefore the spring according to theinvention shows a hysteresis smaller than the hysteresis of the knownspring. Therefore springs according to the invention will be suitable inmany more applications than the known similar springs, particularlywhere precision springs are needed in measuring instruments, where anextremely small hysteresis is important.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawings, of which:

FIG. 1 shows a spring composed of two spring parts and a cylinder shapedprojection at each end, the two spring parts in cross section being halfwave or U-shaped in opposite directions,

FIG. 2 shows a spring corresponding to the spring shown in FIG. 1, butwhere each crest of the half wave and the bottom of the U is of acylindrical shape,

FIG. 3 shows a spring where the two U-shaped parts together form anelliptically ring or toroid shaped tube provided with two cylindershaped projections,

FIG. 4 shows a spring according to the invention composed of nine springparts and a cylinder shaped projection at each end, the spring parts incross section being half wave or U-shaped in opposite directions, thespring parts at the ends being slightly smaller than the other springparts,

FIG. 5 shows a measuring instrument with a weighing cell in which aspring according to the invention has been utilized,

FIG. 6 shows a one and a half wave spring consisting of two radialinwardly directed wave crests and one outwardly directed wave crest,which spring has a cylindrical neutral surface and a cylindrical surfacecontaining the centres of gravity of the cross sections,

FIGS. 7, 8 and 9 show schematically the surfaces containing the centresof gravity for a wave shaped spring formed as a truncated cone, an hourglass and a vase, respectively.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a spring 14A composed of two integrated U-shaped parts 2,2A which, integrated herewith as well, each are provided with acylindrical projection 3. The wave or U-shaped parts 2 and 2A eachcomprises two disc shaped parts 1 and 1A respectively, where the part 2Aforms an outer annular groove and an inner annular wave crest, while theU-part 2 forms an outer annular wave crest and an inner annular groove.

A disc shaped part 1 in continuation of a disc shaped part 1A forms adisc shaped part 5, which via wave crests is integrately connected tothe two other disc shaped parts 1, 1A, each of which carries aprojection 3.

FIG. 2 shows a spring 14B corresponding to the spring shown in FIG. 1,but where each of the disc shaped parts 1, 5 and 5, 1A respectively isintegrally connected to cylinder shaped parts 7 and 9, respectively. Allthe above mentioned disc shaped parts 1, 1A, 5 are flat or slightly coneshaped, which also includes slightly dome shaped forms, and at a zerospring load they form an angle to the spring axis 12 of 90°±5°,preferably 86.4°+0.5° or -1.0°. Each disc shaped part is eitherintegrally connected to the next disc shaped part 5 of the same kind andsize or is integrally connected to a smaller disc shaped part 1, 1A,which has a cylinder shaped projection 3 at its free inner or outeredge. The integral connection may be direct as shown in FIG. 1 as wellas indirect as shown in FIG. 2, as an example via a cylindrical part 7,9.

FIG. 3 shows a ring shaped spring 14C with a tube shaped cross sectionand with opposite and integrally connected projections 3. The tubeshaped cross section is composed of slightly domed disc shaped parts 1,1A. The projections 3 are in the embodiments shown in FIGS. 1-6preferably placed in the same distance from the spring axis 12 as thespring neutral surface 4, which in said cases are a cylindrical surface.

FIG. 4 shows a spring 14D having a spring travel which is larger thanthe travel of the embodiments shown in FIGS. 1-3, but the constructionis based on the same principle. In a practical embodiment of the springthe pitch of the spring may be 12.5 mm, the material thickness of thespring parts 2.75 mm, the inner and outer diameter of the spring partsmay be 45.5 mm and 78.5 mm, respectively, the diameter of the cylindershaped neutral surface may be 62 mm, the inner and outer diameter of theprojections 3 may be 58 mm and 66 mm, respectively. The height of thespring shown in FIG. 4 may e.g. be 67 mm, and the roundings in theintegrated part between two disc shaped neighbouring parts 5 may have aninner radius of curvature of 1.5 mm and an outer radius of curvature of4.25 mm. The material may be temperable steel, so that the steel may behardened to a desired degree of hardness, when a turn out operation andcutting by cutting tools by means of

FIG. 5 shows a weighing cell, in which a spring 14E according to theinvention has been placed between the house 16 of the weighing cell andthe part 18, which carries one of the eye hooks 20 of the weighing cell.The piston rod of the eye hook 20 may be surrounded by a bushing 22,which may be omitted, and which piston rod may be tightened outwardly bya lip sealing 24. The house 16 consists of several parts and sealed by agasket 26.

It has appeared that by application it is possible practically to avoidthe hysteresis loop which normally appears at disc springs, and thiseffect presumably is due to the spring not being exposed to thefriction, which appears between the mutual contact faces of the discsprings and between the disc springs and their guiding means.

FIG. 6 shows a spring 14F with about three half wave formed spring partscomposed of disc shaped parts 1, 1A, 5 and two end projections 3 havinga larger material thickness than the thickness of the wave formed springparts. The material may be metal, steel, glass, plastic or reinforcedplastic, e.g. reinforced by means of whiskers, since the Young's modulusE should only be higher than about 39 GPa. For information it is statedthat the Young's modulus of steel is about 206 GPa, of quartz 72 GPa andof whiskers produced of Al₂ O₃ 524 GPa. Experiments have shown that withan elastic material having an E value higher than 39 GPa excellentresults are achieved as regards to a spring characteristic which ispractically free of hysteresis within the working range of the spring.

For all the springs 14 the line 6 for centres of gravity is shown forthe cross section of the spring. For cylindrical springs of wave shapedform as shown in FIGS. 1-6 this line 6 forms the generatrix of acylindrical surface containing the centres of gravity of the crosssections. Slightly inside in radial direction of said cylindricalsurface is in each of the shown springs a generatrix of the cylinderformed neutral surface of the for bending stresses in relation to whichsurface the bending stresses occurring radially outside the surface areas high as and oppositely directed to the bending stresses occuringradially inside said surface during an increasing, a decreasing or aconstant load of the spring.

Favourable results are also obtained by strong springs 14 with amaterial thickness W which constitutes a relatively high percentage ofthe diameter Y of the spring, or of the diameter N of the tensionneutral surface 4 or of the diameter T of the surface 6 containing thecentres of gravity of the cross sections. Preferably, the ratio P=W:N inthe whole length of the spring is selected in the range 1.5%≦P≦15%, inwhich W is the average material thickness, and N is the diameter of theneutral surface of the spring 14 in that part of the spring length wherethe average material thickness is W.

Such springs 14 could e.g. have a thickness of material of 4 mm and adiameter N of the neutral surface 4 of 26 mm to 270 mm and/or a diameterT of the surface 6 containing the centres of gravity of the crosssections of 28-300 mm. The axial end projections 3 may vary from thesame thickness W of material as the adjacent spring part 2, 2A up toe.g. the double value of said thickness.

In some cases it has appeared appropriate having one additional halfwave formed spring part 2A radially inside the neutral surface 4 of thespring 14. The lowest number of half wave formed spring parts is twowith one half wave formed spring part 2A inside and one half wave formedspring part 2 outside the neutral surface.

FIGS. 7-9 show schematically three different springs which similarly tothe springs shown in FIGS. 1-6 have wave shaped walls, namely a springshaped as a frustum of a cone, as an hour glass and as a vase. The dashand dot lines illustrate the cross section gravity lines for each springseen in an axial plane traversing the two opposite cross sections of thespring in question. Said springs also constitute embodiments of thepresent invention. Since the neutral force surfaces extend parallel tothe surfaces of said gravity lines, said neutral surfaces will also beshaped as shown in FIGS. 7 through 9 as a frustrum of a cone, anhour-glass and a vase, respectively.

I claim:
 1. A spring for compression and tension comprising:asubstantially cylindrical hollow body having a central longitudinalaxis, opposite ends and a wall between said ends of substantiallyconstant thickness and comprising at least two radially directedsubstantially U-shaped sections integrated to form spring parts andbeing symmetrical about said axis and functioning substantially in theaxial direction, said U-shaped sections forming in cross-sectionradially outer and inner wave crests at respective inner and outerdiameters, and said body being made of a material selected from thegroup consisting of metal, steel, glass, plastic, and reinforcedplastic, and having a Young's modulus E of at least 39 GPa; and an endsection at each end comprising an annular rigid axial projectionextending between said inner and outer wave-crest diameters so that aneutral surface of the spring passes through said end sections.
 2. Aspring as claimed in claim 1 wherein:said wall has an average thickness;and the ratio P=W:N for the entire length of the spring is in the range1.5%≦P≧15%, where W is the average wall thickness and N is the diameterof said neutral surface in that part of the spring length having thethickness W.
 3. A spring as claimed in claim 2 wherein said neutralsurface comprises a cylindrical surface.
 4. A spring as claimed in claim3 wherein the number of U-shaped sections extending radially outwardlyof said neutral surface is one less than the number of U-shaped sectionsextending radially inwardly of said neutral surface.
 5. A spring asclaimed in claim 3 comprising one U-shaped section extending radiallyoutwardly of said neutral surface and two U-shaped sections extendingradially inwardly of said neutral surface.
 6. A spring as claimed inclaim 1 wherein:each spring part at said neutral surface forms an angleto said central axis of 90°±5°.
 7. A spring as claimed in claim 1wherein said neutral surface comprises a cylindrical surface.
 8. Aspring as claimed in claim 1 wherein said neutral surface comprises aconical surface.
 9. A spring as claimed in claim 1 wherein said neutralsurface comprises a double conical surface, said conical surfaces havingopposite inclinations so that said surfaces intersect each other.
 10. Aspring as claimed in claim 1 wherein the number of U-shaped sectionsextending radially outwardly of said neutral surface is one less thanthe number of U-shaped sections extending radially inwardly of saidneutral surface.
 11. A spring as claimed in claim 10 comprising oneU-shaped section extending radially outwardly of said neutral surfaceand two U-shaped sections extending radially inwardly of said neutralsurface.
 12. A spring as claimed in claim 1 comprising two U-shapedsections oppositely directed so that both sections extend radiallyoutwardly of said neutral surface and are integrated to form a slightlyflattened toroid.
 13. A spring as claimed in claim 1 wherein:each springpart at said neutral surface forms an angle to said central axis of86.4°±0.5°/-0.1°.